In the light of increasingly rapid climate change worldwide, one of the primary
goals is to reduce financial losses of dairy farmers and to enable the sustainable
farming. In order to realize those goals, the necessity of implementation of breeding
values for heat resistance in breeding strategies, have become more and more
pronounced. Estimation of breeding values requires determination of temperaturehumidity
index (THI) threshold value. Therefore, the objective of this research was
to determine the THI threshold value for the first parity Holsteins in environmental
conditions in Eastern Croatia. With that purpose individual test-day records of the
first parity Holsteins with records of ambient temperature and relative humidity
in the barns were analysed. Data were collected in regular milk recording from
January 2006 to December 2012. The THI threshold values for daily milk yield were
determined by least square analyses of variance for each given THI value (from 65
to 76) using the PROC MIXED (SAS). The THI <68 did not cause significant change
in daily milk production of the first parity Holsteins. Significant decrease of daily
milk yield was observed at THI ≥ 68 with estimated drop from 0.240 to 0.716 kg
milk/day (THI from 68 to 76). The THI=68, as the lowest value at which significant
decrease in daily milk yield was determined, was taken as the threshold value for
the first parity Holsteins in Eastern Croatia.